1. Field of the Invention
The invention relates to a toothed belt for power transmission with tooth root portions strengthened by a filament thread twined around a core thread, and a method of manufacturing the toothed belt.
2. Description of the Related Art
Conventionally, for example, a toothed belt is widely adopted as a member for transmitting power between two shafts in a vehicular engine, a machine tool, or the like. In particular, a toothed belt used for an engine is also called “a timing belt”.
In general, a toothed belt is formed of synthetic rubber or polyurethane rubber. Being lighter and more flexible than a metallic roller chain, this toothed belt boasts of its low noise level even during rotation, and besides, does not require any lubricating operation. On the other hand, a raw material itself from which the toothed belt is made exhibits low strength, and cuts tend to appear in the toothed belt due to a deterioration of the raw material. Therefore, the toothed belt also faces some problems concerning rigidity and durability.
Thus, in the conventional toothed belt, a core wire made of, for example, glass fiber or aramid fiber is embedded in the toothed belt, and surfaces of belt tooth portions are reinforced by a tooth cloth piece made of nylon woven fabric to restrain the toothed belt from stretching in a longitudinal direction thereof. In this manner, the rigidity of the toothed belt is enhanced as a whole.
Now, the aforementioned conventional toothed belt will be described with reference to FIG. 5. In FIG. 5, belt tooth portions T and belt groove portions D are alternately formed along a toothed belt B. A core wire C obtained by intertwining a plurality of strands of aramid fiber is embedded in the toothed belt B, and a tooth tip-side surface of the toothed belt B is covered with a tooth cloth piece S. The belt tooth portions T, the belt groove portions D, the core wire C, and the tooth cloth piece S are integrally formed by a belt material G.
However, in the aforementioned toothed belt B, when a force F is applied to the belt tooth portions T during power transmission, the belt tooth portions T are so deformed as to tumble in a direction of the force F, and a stress concentrates on boundary portions P of tooth root portions serving as boundaries between the belt tooth portions T and the belt groove portions D respectively. Then, when the stress reaches a predetermined stress, cuts are made in the boundary portions P. Shear fracture progresses from these cuts toward boundary portions Q on the other side of the belt tooth portions T (as indicated by broken lines L), and the belt tooth portions are chipped above the boundary portions Q. Accordingly, it has been desired to develop a toothed belt capable of restraining belt tooth portions from being deformed under a high-load condition as well and effectively inhibiting the belt tooth portions from being damaged.
In view of the aforementioned problem, toothed belts with strengthened tooth root portions are disclosed in Japanese Patent Application Publication No. 50-69452 (JP-A-50-69452), Japanese Utility Model Application Publication No. 63-137149 (JP-U-63-137149), and Japanese Patent Application Publication No. 7-156288 (JP-A-7-156288). In the toothed belt disclosed in Japanese Patent Application Publication No. 50-69452 (JP-A-50-69452), a core wire embedded in the toothed belt is fabricated from a multifilament, and ends of a plurality of filaments of the multifilament are so arranged as to protrude outward from an outer peripheral face of the multifilament to increase an area of adhesion to rubber. As a result, the toothed belt is strengthened as a whole. Further, in the toothed belt disclosed in Japanese Utility Model Application Publication No. 63-137149 (JP-U-63-137149), a core wire for reinforcing belt tooth portions, which has a curved portion to reinforce the belt tooth portions, is embedded together with a core in a belt to strengthen the belt tooth portions. Further, in the toothed belt disclosed in Japanese Patent Application Publication No. 7-156288 (JP-A-7-156288), a nonwoven fabric is press-fitted into tooth root portions using a pressure generated by the winding of a core wire, and discontinuous fiber contained in the nonwoven fabric is diffused to the tooth root portions during molding, so that the tooth root portions of the belt are strengthened.
In the toothed belt disclosed in Japanese Patent Application Publication No. 50-69452 (JP-A-50-69452), the ends of the plurality of the filaments cannot be protruded homogeneously in a longitudinal direction of the toothed belt, and the core wire is inhomogeneous in strength in the longitudinal direction. Thus, the toothed belt is inhomogeneous in strength as a whole. In addition, the multifilament as described above is complicated to fabricate, and causes a steep rise in manufacturing cost. Further, in the toothed belt disclosed in Japanese Utility Model Application Publication No. 63-137149 (JP-U-63-137149), when being fabricated together with (twined around) the core wire, the core wire for reinforcing the belt tooth portions needs to be fabricated in accordance with the pitch and dimension of the belt tooth portions. Furthermore, the core wire for reinforcing the belt tooth portions is arranged along outer peripheral faces of the belt tooth portions, and hence causes an increase in rigidity not only in the tooth root portions, which need to be reinforced, but also in the entire belt tooth portions including tooth tip portions. As a result, new problems such as noise and the like may arise. Further, in the toothed belt disclosed in Japanese Patent Application Publication No. 7-156288 (JP-A-7-156288), the nonwoven fabric needs to be arranged in accordance with the pitch of the belt tooth portions as is the case with the aforementioned Japanese Utility Model Application Publication No. 63-137149 (JP-U-63-137149). Furthermore, it is difficult to control the arrangement of the discontinuous fiber of the nonwoven fabric that is diffused to the belt tooth portions as polyurethane flows during molding. Consequently, the tooth root portions of the toothed belt cannot be effectively strengthened.